explain

This is intended to be used in developing Similarity implementations,
and, for good performance, should not be displayed with every hit.
Computing an explanation is as expensive as executing the query over the
entire index.

doc

Get the Document at the nth position. The FieldSelector
may be used to determine what Fields to load and how they should be loaded.
NOTE: If the underlying Reader (more specifically, the underlying FieldsReader) is closed before the lazy Field is
loaded an exception may be thrown. If you want the value of a lazy Field to be available after closing you must
explicitly load it or fetch the Document again with a new loader.

This is intended to be used in developing Similarity implementations,
and, for good performance, should not be displayed with every hit.
Computing an explanation is as expensive as executing the query over the
entire index.